CO oxidation on TiO₂ (110) supported subnanometer gold clusters: Size and shape effects

We performed a comprehensive study of catalytic activities of subnanometer Au clusters supported on TiO₂(110) surface (Au_n/TiO ₂, n = 1-4, 7, 16-20) by means of density functional theory (DFT) calculations and microkinetics analysis. The creditability of the chosen DFT/microkienetics methodologies was demonstrated by the very good agreement between predicted catalytic activities with experimental measurement (J. Am. Chem. Soc, 2004, 126, 5682-5483) for the Au_1-4/TiO₂ and Au₇/TiO₂ benchmark systems. For the first time, the size- and shape-dependent catalytic activities of the subnanometer Au clusters (Au₁₆-Au₂₀) on TiO₂ supports were systematically investigated. We found that catalytic activities of the Au _n/TiO₂ systems increase with the size n up to Au ₁₈, for which the hollow-cage Au₁₈ isomer exhibits highest activity for the CO oxidation, with a reaction rate ∼30 times higher than that of Au₇/TiO₂ system. In stark contrast, the pyramidal isomer of Au₁₈ exhibits much lower activity comparable to the Au _3-4/TiO₂ systems. Moreover, we found that the hollow-cage Au₁₈ is robust upon the soft-landing with an impact velocity of 200 m/s to the TiO₂ substrate, and also exhibits thermal stability upon CO and O₂ co-adsorption. The larger pyramidal Au₁₉ and Au₂₀ clusters (on the TiO₂ support) display much lower reaction rates than the pyramidal Au₁₈. Results of rate of reactions for unsupported (gas-phase) and supported Au clusters can be correlated by a contour plot that illustrates the dependence of the reaction rates on the CO and O₂ adsorption energies. With the TiO₂ support, however, the catalytic activities can be greatly enhanced due to the weaker adsorption of CO on the TiO₂ support than on the Au clusters, thereby not only the ratio of O₂/CO adsorption energy and the probability for the O ₂ to occupy the Ti sites are increased but also the requirement for meeting the critical line becomes weaker. The obtained contour plot not only can provide guidance for the theoretical investigation of catalytic activity on other metal cluster/support systems, but also assist experimental design of optimal metal cluster/support systems to achieve higher catalytic efficiency.